carbon filtration removed volatile organic compounds and mercury to levels compliant with the state drinking water standards, though they also noted the water used may already need to be "exceptionally clean" prior to filtration. Limitations Copper zinc water filtration does not remove organic chemicals, such as pesticides and disinfection byproducts, nor is it effective against the parasitic cysts of giardia or cryptosporidium. and must be periodically backwashed with hot water to clean them. This reduces their efficiency, and the pollutants dislodged by washing can lead to water contamination. The United States Environmental Protection Agency found that copper zinc water filtration can remove mercury from contaminated water, but only at low concentrations, and recommends that for highly contaminated water other processes be used. Due to their bactericidal action, copper zinc water filtration devices are considered by the EPA to be "pesticidal". However, Stanford physician Paul Auerbach recommends against their usage as a sole means of germicidal water treatment, and he does not include them amongst his recommended protozoal disinfection methods at either point of entry or point of use. A 1995 United States Environmental Protection Agency report found that such systems were employed at approximately 20 US-based cooling towers in 1993. The report documented variable results, with some systems discontinued because they were ineffective at controlling bacterial growth, though in other instances they were preferred because of comparatively safe waste production and simpler maintenance. There is also concern about environmental damage due to the release of zinc in areas with high concentrations of metals or certain pollutants, in particular copper and chlorine. Publications of the American Water Works Association do not recommend the use of copper zinc water filtration systems to treat chlorinated water that outflows to streams. Studies have also shown that regulation standards for the systems can vary widely or be nonexistent depending on the industry and region of their usage. See also Antimicrobial properties of copper Antimicrobial copper-alloy touch surfaces Chloramine References Further reading Materials safety data sheet published by General Electric. Water treatment Copper alloys Zinc alloys Brass Water filters